PTL 1 discloses a technology relating to migrating file data between two servers (file servers), and describes a system including a client, a first server having a primary storage, a second server having a secondary storage, and a control device. In this system, a stub file in which location information of a real file migrated from the primary storage to the secondary storage has been recorded is located in the primary storage. Upon receipt of a file access request to the primary storage of the first server from the client, the control device determines whether the access target file is a stub file. When determining that the access target file is a stub file and that the file access request requires access to the real file, the control device accesses to the real file in the secondary storage based on the information of the stub file and returns a response to the client.
According to PTL 1, a file stubbing is triggered when the storage capacity reaches a threshold, stubbing is performed from a file that has not been accessed, and a file accessed at an earlier time is migrated to a lower layer according to an assigned quota value.